Ignition of a fuel-air mixture in the combustion chamber of an internal combustion engine (ICE) of the Otto-type is done by means of a sparkplug in which a high-voltage spark, for example generated by discharge of a capacitor, is caused to discharge across a firing or spark gap of the sparkplug. The capacitor, or another engergy storage device such as an ignition coil itself, is charged with energy and, at a predetermined time instant which may be controlled by a computer, the capacitor or other energy storage device discharges causing the spark to flash over at the spark gap. The spark gap ignites the combustible mixture within the combustion chamber of the ICE.
Timing of the spark in relation to the combustible charge, and the position of a piston in the ICE, usually taken with reference to the top dead-center (TDC) position of the piston, is important. The spark flash over is usually caused to occur at a predetermined time instant in advance of (TDC) position of the piston so that the mixture will burn, and give off energy just at and after the piston has reached TDC position, to obtain maximum efficiency from the burning mixture. For most efficient operation, it is important that the mixture should burn as rapidly as possible within the combustion chamber, and that a frontal zone of combustion, or flaming, of the combusitible mixture propagates as rapidly as possible.
The electrical discharge which occurs at the spark gap of the sparkplug under control of the associated ignition system is, unfortunately, not a clearly analyzable occurrence or event as, for example, an electrical square-wave pulse or the like which controls the discharge. As the spark gap forms, three phases can be distinguished:
(1) the breakdown phase;
(2) the arcing phase; and
(3) the glow phase.
The energy transferred in the various ones of the phases differs greatly. The formation of the respective phases depends to some extent on the geometry of the ignition electrodes, as well as on the associated circuitry connected thereto. If the ignition system provides a high-voltage pulse to the ignition electrodes, then, first, after the breakdown voltage has been exceeded, an electically conductive plasma path will result. The curents which flow through the path between the electrodes may be high. This occurs during phase (1), that is, the breakdown phase.
The next phase is the arcing phase, the formation and course of which depends to some extent on the circuitry with which the sparkplug is associated. The arcing phase causes current to flow in the previously generated plasma path. The voltage between the electrodes may be comparatively low or the current which flows at the beginning of the second, or arcing phase may be very high. When the current during the arcing phase drops below a predetermined value, the arc will extinguish. The third, or glow phase will follow. The current during the third or glow phase also flows through the plasma which has previously been generated. The voltage is above the value of the voltage during the arcing phase.
The sparkplug is stressed differentially during the respective phases. In the breakdown phase, the heat loading on the sparkplug is low. In the arcing phase, the heat loading is high, and heat which is applied to the ignition elecrodes of the sparkplug leads to the well-known erosion and deterioration of the sparkplug.
It has been found that the volume of the mixture of combustible gases which is activated and then ignited by the flash-over of the spark during the sparking phase is highest during the breakdown phase, and higher than during the arcing phase or the glow phase of the overall discharge of the sprakplug. Thus, the breakdown phase causes the most rapid reaction of the combustible mixture to the spark. The reliability of ignition, that is, the assurance that the spark will cause ignition of the combustible mixture is highest in the breakdown phase, and higher than during the arcing and the glow phase. If the mixture has not ignited during the breakdown phase of the spark, it may not ignite during the subsequent phase even if some plasma has formed.
In order to ensure reliable ignition, it is desirable that as much of the ignition energy as possible should be supplied during the breakdown phase of the sparkplug arc. Various arrangements to accomplish this aim have been proposed, see, for example, German patent publication document DE-OS No. 28 10 159. As described in this publication, the energy transfer is concentrated during the discharge phase, that is, during the initial phase of the spark at the head of the sparkplug. The system stabilizes ignition of the mixture and accelerates the speed, of the flame front which occurs during ignition, in comparison to prior systems and apparatus.
The loading conditions applied to an Otto-type ICE result in different conditions of combustible mixtures in the combustion chamber. Upon full load operation, the mixture is rich and the degree of fill of the combustion chamber is high. Igniting such a mixture does not pose any problems. An accelerated transfer of energy is not even necessarily desired. If the ICE, however, operates at low loading, or under idling condition or, even under engine braking conditions, the temperature within the combustion chamber drops rapidly and the pressure also drops. The mixture is lean, and the degree of fill of the combustion chamber of the ICE is low. Non-homogeneties of the mixture occur, and consequently, ignition of the already lean, and possibly non-homogenous and insufficiently filled, mixture may cause difficulties.